When a girl wears clothes that are too small, particularly jeans. Tight jeans push up the fat on her hips, which spills over the sides of the pants like the top of a muffin around the wrapper.
In certain cultures, especially in military tours to Afghanistan, asking for a 'blueberry muffin' at a massage parlor means to get a hand job at the end in exchange for a big tip.
person 1: I got a massage today at FOB Shank massage parlor.
person 2: Did you get a Blueberry muffin at the end?
person 1: Yeah, it cost me an extra $20.